Image guides are bundles of optical fibers which convey optical images. Because each optical fiber of an image guide transmits only a minute discrete portion of the image, it is of course necessary for each end of the image guide to be coherently related to the other end such that the image exiting the image guide is identical to that which enters the multiplicity of fibers. Image guides are used in a variety of industrial and medical imaging scopes. For example, endoscopes utilize image guides to convey images of human and/or animal vessels and internal cavities. Additionally, image guides are also used in industrial borescopes used for many types of industrial imaging.
Image quality is critical to the performance of image guides. Specifically, resolution, brightness, and contrast sensitivity are a few important performance characteristics which affect image quality. Resolution can be expressed as the measure of the image guide's ability to separate images of two neighboring object points. Improved image resolution can be obtained by having a larger number of optical fibers, in the bundle, per unit area. The brightness of an image guide is a measure of the ratio of the amount of light exiting the output end of the image guide to the amount of light incident to the input end of the image guide. The brightness of an image guide can be improved by, for example, increasing the portion of the image guide end available for light transmission, increasing the numerical aperture (NA), and/or decreasing the transmission loss of the image guide. The contrast sensitivity is a measure of the ratio of the amount of light, comprising the image, exiting the output end of the image guide to the total amount light exiting the output end of the image guide. The light exiting the output end of the image guide, and not contributing to the image, reduces the contrast sensitivity.
Depending upon the intended use of the image guide, other characteristics such as flexibility may also be important. For example, it is often advantageous for image guides to have great flexibility to reach otherwise inaccessible locations such as coronary vessels. In other applications, such as laparoscopy, a more rigid image guide is preferred. The subject invention concerns, in one aspect, improved image guides which result in endoscopes and borescopes with highly advantageous characteristics.
One specific embodiment of the subject invention is the use of improved image guides in angioscopes. Angioscopy is a specific type of endoscopy which uses a flexible angioscope to transmit images from the heart and the coronary tree. Angioscopes are valuable tools for use in the investigation and treatment of heart and vascular disease. In various studies, atheromatous plaque rupture and splitting, endothelial exfoliation, and thin mural thrombi that could not be detected by angiography were able to be detected by angioscopy (Ushida, Y. et al. 1989! Am. Heart Jourmal 117(4):769-776). Unfortunately, angioscopes, which are typically between 1.0 and 1.5 mm in diameter, are not small enough to access the entire coronary tree.
The image guide of existing angioscopes typically has a diameter of about 0.27 mm and is surrounded by fibers arranged circumferentially to provide uniform illumination of the inner lumen. FIG. 1 is a schematic structure of an angiofiberscope image guide. An angioscope image guide is typically a hexagonal array of about 2000 fibers of the step index type. A step index (SI) optical fiber is one in which a fiber is composed of a core surrounded by a cladding where the refractive indices of the core and cladding are n.sub.1 and n.sub.2 respectively, where n.sub.1 &gt;n.sub.2. Typically, this SI optical fiber is glass but, as discussed below, SI polymer optical fiber is also known. Light at less than the critical angle, which is transmitted down the core experiences internal reflection with very high efficiency at the core/cladding interface. Although the light reflects efficiently at the boundary, a small fraction of the light temporarily penetrates the cladding in the form of evanescent waves before returning to the core. If the cladding is not thick enough, these evanescent waves can pass through the cladding causing some of this light to leak out, or tunnel, through the cladding into the adjacent fiber. This causes a reduction in resolution and a reduction in contrast sensitivity. If the core diameter is reduced, at fixed cladding thickness, less light is transmitted and the image loses brightness. On the other hand, if the cladding thickness is reduced, for a fixed core diameter, more leakage, or tunneling, occurs. Hence, there is an optimum fiber core diameter and cladding thickness. This optimization process has been studied experimentally (Tsumanuma, T. et al. 1988! Proc. SPIE 906:92-96). Tsumanuma et al. determined that a core diameter of 3 .mu.m and cladding thickness of 1 .mu.m was optimal.
With a core diameter of 3 .mu.m and a cladding thickness of 1 .mu.m, only 36% of the light which hits the end of the step-index glass fiber image guide actually strikes the area defined by the cores of the microfibers. Most of the available light is lost on the cladding area. Since it is only light striking the core area which can contribute to image brightness, only a marginal reduction in microfiber diameter can be made without significant brightness reduction.
The resolution of an image guide is dependent on the number of microfibers per unit cross-sectional area. For example, existing angioscope image guides cannot be increased significantly in diameter to incorporate more microfibers, due to the dimensions of the vascular system, and the diameter of the presently employed microfibers cannot be reduced in size without significant brightness reduction. Therefore, it is difficult to improve the resolution of existing angioscopes.
Another important characteristic of flexible image guides is flexibility as measured by the minimum bend radius of the image guide. The flexibility of existing angioscopes is typically limited by the stiffness of the image guide. For example, the typical minimum bend radius is about 8 mm, which makes procedures difficult in some regions of the coronary tree. This degree of flexibility has been achieved by acid leaching of the image guide to divide it into several separate units, except for its ends where the nicrofiber spatial coherence is mandatory. Further subdivision of the glass image guide would increase flexibility, but at the expense of rapidly increasing the fragility of the microfibers. There is already a fairly rapid deterioration of image quality due to microfiber breakage which shows up as black spots on the image. In addition, coloration of the transmitted image of glass endoscopes has been observed (Tsumanurna et al, supra) when the endoscope is subjected to severe bending as occurs in angioscopy. This can cause loss of spectroscopic information in angioscopic clinical diagnosis due to wavelength dependent light leakage from the fiber cores.
Many image guides are made with step index glass optical fiber. Polymer optical fiber fabricated with a step index (SI) of refraction is also known to those in this art. A cross section of such an SI fiber is shown in FIG. 2. Both polymer and glass SI fibers are constructed with a core and cladding with refractive indices n.sub.1 and n.sub.2 respectively, where n.sub.1 &gt;n.sub.2. A second type of fiber is known as gradient-index or graded-index (GRIN) fiber can also be made with polymer or glass. The GRIN structure is also shown in FIG. 4.
In comparing the SI structure with the GRIN structure, it is noted that there are different trajectories of light rays in these two fiber structures. This is shown schematically in FIG. 5. Within SI fiber, the light travels in straight lines. At angles less than the critical angle of internal reflection, the light is reflected at the core cladding interface. At angles greater than the critical angle, the light is refracted into the cladding from which it travels into the adjacent fiber in the SI image guide. This large angle light traverses the various fibers in the image guide until it reaches the side of the image guide and is absorbed. In contrast, within GRIN fiber, the light travels in a curved trajectory, always being refracted back towards the axis of the fiber. At angles less than the critical angle, light never reaches the outer edge of the fiber. At angles greater than the critical angle, the light exits the fiber similar to the case of the SI image guide.